1. Field of the Invention
The present invention relates to a radar for detecting vehicles, people, and so on by using, for example, radio waves in the millimeter-wave band.
2. Description of the Related Art
Hitherto, a continuous-wave radar is known as a radar for mounting on vehicles such as automobiles. This continuous-wave radar frequency-modulates a transmission signal, transmits the signal, and receives a reflection signal from a target after transmission, so as to measure the distance to the target and the relative velocity of the target by using a beat signal between the transmission signal and the reception signal. In this radar using a modulated continuous-wave, the following problems occur.
That is, the frequency of the beat signal, which is a frequency difference signal between the transmission signal and the reception signal, decreases as the distance between the radar and the target decreases. When the frequency of the beat signal decreases to two periods or less in an observation range, the resolving power of the radar becomes less than the resolving power of a fast Fourier transform (FFT). As a result, the peak frequency cannot be detected accurately and thus the distance is difficult to measure. Also, since the frequency component of the beat signal caused by the reflection signal from such a close range appears in a region of large noise, in the vicinity of the DC component, the detecting performance for a close range deteriorates due to the relationship with a noise index of a reception system. For these reasons, the detection coverage becomes narrow.
In (1) Japanese Unexamined Utility Model Registration Application Publication No. 5-50383 and (2) Japanese Unexamined Patent Application Publication No. 11-109026, attempts to overcome the above-described problems are disclosed.
In the radar disclosed in (1), a beat signal corresponding to a close range less than the FFT resolving power is effectively used. That is, it is determined that a target exists within a close range when the spectrum level of the DC component and the frequency components adjacent thereto among the frequency components obtained by the frequency analysis of a beat signal increases so as to exceed a predetermined value in the case where a target does not exist.
In the radar disclosed in (2), the frequency of any one of a reception signal and a local oscillation signal for generating a beat signal to be input to a mixer is offset by an intermediate frequency.
Also, in (3) Japanese Unexamined Patent Application Publication No. 10-253750, detection errors and calculation errors of a relative velocity and a relative distance are prevented by removing the effect of offset voltages of a transmitter-receiver and an AD converter, from a point of view other than improving the close range detection performance.
However, the DC component appears in the output of a mixer even when a target does not exist in a close range. Also, the DC component appears when extracting samples for FFT operation asynchronously with respect to a signal period. Therefore, in the radar according to (1), no significant variation in the spectrum level of the DC component and the nearby frequency components of a beat signal appears in the case where a target exists in a close range and in the opposite case. As a result, it is difficult to set a threshold for determining the existence of a target.
In the radar according to (2), a circuit for offsetting the frequency of a beat signal by the intermediate frequency is required. Also, frequency analysis must be performed over a large frequency range because of the offset. Thus, a sampling frequency must be increased and a high-rate processing ability is required accordingly.
In the radar according to (3), the average of a beat signal between a transmission signal and a reception signal is obtained as an offset component by stopping transmission so as to calculate an offset voltage generated in the circuit of the transmitter-receiver and an offset voltage generated in the circuit of the AD converter. The offset component is compensated steadily to an AD conversion value.
However, even when the offset voltage generated in the circuit of the transmitter-receiver and the offset voltage generated in the circuit of the AD converter are steadily compensated, by dividing sampling data to be frequency-analyzed by FFT into necessary sampling intervals asynchronously with respect to the period of the beat signal, a DC component appears in the FFT result due to so-called truncation. Also, the DC component appears for the same reason when the beat signal is less than one range bin (frequency resolving power) of the FFT, that is, when the beat signal is less than one period in the sampling interval for the FFT, due to the reflection of a signal in the circuit included in the radar.
Consequently, the DC component cannot be removed from the FFT result simply by compensating the offset voltage generated in the circuit of the transmitter-receiver and the offset voltage generated in the circuit of the AD converter.
These problems occur when velocity measurement in a low-velocity region is performed in radars for detecting a Doppler shift frequency, such as a pulse Doppler radar and a frequency shift keying (FSK) radar, as well as a radar using a modulated continuous-wave, such as a frequency modulated continuous wave (FMCW) radar.
Accordingly, it is an object of the present invention to provide a radar by which target detection can be performed easily without having a complex overall configuration, when close range detection is performed by a modulated continuous-wave radar such as an FMCW radar and low-velocity range detection is performed by a radar for detecting a Doppler shift frequency, such as a pulse Doppler radar or an FSK radar.
In order to achieve the above-described object, a radar according to the present invention comprises a unit for transmitting a transmission signal as a detecting radio wave and receiving a reception signal including a reflection signal from a target which reflects the transmission signal as the reception signal; a sampling-data-sequence generating unit for sampling a beat signal which is a signal of the frequency difference between the transmission signal and the reception signal and AD-converting the beat signal so as to obtain a sampling data-sequence having a predetermined number of data items; an analyzed data generating unit for generating data to be frequency-analyzed by subtracting the average of the data in a predetermined sampling interval to be frequency-analyzed of the sampling data-sequence from each of the data items in the sampling interval; and a unit for frequency-analyzing the data to be frequency-analyzed by a discrete Fourier transform to obtain the frequency component of the beat signal and for detecting the target based on the frequency component.
With this arrangement, the DC component does not appear in a result of discrete frequency analysis by, for example, fast Fourier transform (FFT), and thus the existence of a peak in a low-frequency region in the vicinity of the DC component can be reliably detected. As a result, a target can be easily detected without a complex overall configuration, even when target detection in a close range is performed by using a modulated continuous-wave radar such as an FMCW radar or when detection in a low-velocity region is performed used by a radar for detecting a Doppler shift frequency, such as a pulse Doppler radar or an FSK radar.
Preferably, the analyzed data generating unit generates the data to be frequency-analyzed by subtracting the average of the data in the sampling data-sequence from each of the data items in the sampling data-sequence and by providing window function processing. That is, the DC component is removed before window function processing. Accordingly, the spectrum in the vicinity of the DC component does not become wide after window function processing, and thus the existence of a peak in a low-frequency region in the vicinity of the DC component can be reliably detected.
Further, in the present invention, the detecting radio wave is a frequency-modulated wave including an up-modulation interval in which the frequency gradually increases and a down-modulation interval in which the frequency gradually decreases, the up-modulation interval and the down-modulation interval being repeated over time so as to form a triangular waveform. The relative distance and the relative velocity of the target are detected based on the beat signal in the up-modulation interval and the beat signal in the down-modulation interval.
With this arrangement, the distance to a target in a close range and the relative velocity can be detected by a so-called FMCW method.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings.